RU2427662C2 - High strength welded steel pipe for pipeline possessing excellent low temperature ductility and procedure for its fabrication - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: in process of steel production it is preliminary deoxidised with Si and Mn, further there is introduced Ti for obtaining composition of steel containing wt %: C from 0.010 to 0.050, Si from 0.01 to 0.50, Mn from 0.50 to 2.00, S from 0.0001 to 0.0050, Ti from 0.003 to 0.030, O from 0.0001 to 0.0080, B from 0.0003 to 0.0030, P 0.050 or less, Al 0.020 or less, Mo to less, than 0.10, when required, one or more from Cu from 0.05 to 1.50, Ni from 0.05 to 5.00, Cr from 0.02 to 1.50, V from 0.010 to 0.100, Nb from 0.001 to 0.200, Zr from 0.0001 to 0.0500, Ta from 0.0001 to 0.0500, Mg from 0.0001 to 0.0100, Ca from 0.0001 to 0.0050, REM 0.0001 to 0.0050, Y from 0.0001 to 0.0050, Hf from 0.0001 to 0.0050, Re from 0.0001 to 0.0050, W from 0.01 to 0.50, iron and unavoidable impurities - the rest. Produced steel is cast into a slab which is heated to temperature 1000°C or higher. The slab is hot rolled to sheet at relative reduction 2.5 or more in the region of temperatures preceding crystallisation corresponding to 900°C or lower. Further, there is performed water cooling stopped at temperature 600°C or lower. The produces sheet is formed into a pipe and butt sections are seam welded on internal and external surfaces. structure of the steel sheet consists of polygonal ferrite occupying 20 % of area or less and bainite occupying 80 % of area or more. Also, actual dimension of a grain in basic material of steel amounts to 20 mcm or less, while actual dimension of a grain in a zone of thermal influence at welding is 150 mcm or less.

EFFECT: high strength and excellent low temperature ductility of steel.

13 cl, 1 dwg, 4 tbl, 1 ex

Description

FIELD OF THE INVENTION

The present invention relates to a high strength welded steel pipe for a pipeline having excellent low temperature viscosity and suitable for a pipeline transporting crude oil and natural gas.

State of the art

Due to the fact that the steel pipe for the pipeline used in the main lines is essential, as well as the methods of transporting crude oil, natural gas and other materials through the pipeline over long distances, a high-strength steel pipe with a high viscosity was proposed for the pipeline (for example, Patent Document 1). Currently, a steel pipe manufactured in accordance with the American Institute of Petroleum (AIN) X70 standard (tensile strength 564 MPa or higher) or a steel pipe with a higher strength category, up to X80 (tensile strength 620 MPa or higher), is used for the pipeline. but, in order to increase the efficiency of transportation of crude oil and natural gas, studies have been conducted related to an increase in internal pressure in pipelines. In addition, the pipeline requires a high-strength steel pipe with a strength category of X70 or higher or, moreover, with a strength category of X80 or higher, having a tensile strength of 600 MPa or higher, to further increase the strength and increase the thickness of the pipe.

If we talk about higher strength, then, for example, when using pipes with a strength category X120 in the pipeline with a tensile strength of 900 MPa or higher, it is possible to increase the internal pressure in the pipeline, i.e. increase the pressure of crude oil or natural gas by about half compared with a pipeline that uses pipes with strength category X65, making it possible to transport approximately twice the amount of crude oil or natural gas. In addition, if you increase the strength of the pipeline and increase the resistance to internal pressure, it becomes possible to reduce the cost of material, transportation costs and the cost of welding in place, while if you increase the thickness of the pipeline, the costs associated with laying the pipeline can significantly increase.

In addition, the pipeline must have excellent low temperature viscosity, as it is often laid in cold regions. In addition, during the laying of the pipeline, the ends of the pipes are joined together, which also requires excellent local weldability. Proposed, for example, in Japanese patent publication JP (A) No. 2004-52104, a steel pipe with a strength category X120 for a pipeline that meets these requirements and with higher strength compared to a steel pipe for a pipeline proposed in Japanese patent publication JP (A ) No. 62-4826. The microstructure of the base material of this high-strength steel pipe for a pipeline consists mainly of a mixture of bainite and martensite. Additionally, to increase the thickness of the pipe, methods are proposed for manufacturing plate steel with a metal structure consisting of finely divided bainite obtained by controlled rolling and controlled cooling, while the steel has good strength and toughness (for example, Japanese patent publication JP (A) No. 2000-256777 , Japanese Patent Publication JP (A) No. 2004-76101 and Japanese Patent Publication JP (A) No. 2004-143509).

The steel pipe for the pipeline is made by giving the plate shape of the pipe in the UO process, and, drawing the edges of the workpiece to each other, welded them by roller welding. When viscosity and reliability are required, such as for a high-strength steel pipe for a pipeline, instead of roller welding, submerged arc welding is preferably used on the inner and outer surfaces of the pipe. When welding steel material repeatedly, a problem arises because the heat affected zone (called “HAZ”), the structure of which is coarsened as a result of heating during primary welding, is re-heated during subsequent welding, and therefore the viscosity decreases.

As a technology for improving the low-temperature viscosity of HAZ of a high-strength steel pipe pipe, a method based on intragranular transformation has been proposed to obtain a finer grain structure in HAZ (for example, Japanese Patent Publication JP (A) No. 8-325635, Japanese Patent Publication JP (A ) No. 2001-355039 and Japanese Patent Publication JP (A) No. 2003-138340). In the method proposed in Japanese patent publication JP (A) No. 8-325635, the formation of acicular ferrite, which is the nucleus for the crystallization of oxides, occurs. In accordance with the methods proposed in Japanese Patent Publication JP (A) No. 2001-355039 and Japanese Patent Publication JP (A) No. 2003-138340, intragranular bainite is formed using a mixture consisting of oxides and sulfides as crystallization nuclei.

With an increase in the Mo content, the hardenability of most conventional high-strength steel pipes used for pipelines is improved, which is effective for increasing strength, and mainly a bainitic metal structure is formed, which contributes to an increase in toughness, but currently there is a tendency to decrease the content of an expensive element in steel Mo. However, with a decrease in the Mo content, hardenability somewhat decreases and it becomes difficult to obtain intragranular bainite, and therefore it is difficult to provide a low-temperature viscosity of HAZ. However, the maximum thickness of a conventional high-strength pipeline is less than 25 mm. There is a need for pipelines with a thickness of 25 mm or more or 30 mm or more.

Disclosure of invention

The present invention provides an inexpensive high-strength welded steel pipe for piping having excellent low temperature viscosity, and low temperature viscosity of HAZ can be provided, in particular, even with a limited Mo content, and a method for its manufacture is proposed.

For the manufacture of the pipeline, the authors produced plate steel with a strength category of X70 or X80 or more, with a sheet thickness of 25 mm or more and a tensile strength (PR) of 600 MPa or more. As a result, it was found that the problems associated with the increase in plate thickness are much more serious than expected. In particular, with controlled rolling and an adjustable cooling rate, sufficient rolling of the central part over the sheet thickness is not ensured, and therefore its viscosity decreases markedly with respect to the viscosity of the surface layer of the steel sheet. The authors additionally investigated the metal structure in the central part by the thickness of the steel sheet, as a result of which it was found that it is extremely difficult to create a finely dispersed bainitic structure in the central part of the sheet thickness of high-strength plate steel.

The present invention solved the above problems and proposed an inexpensive thick-walled high-strength welded steel pipe for piping having excellent low-temperature viscosity, in which it is possible to limit the Mo content, even with a pipe thickness of 25 mm or more or 30 mm or more, and also a method for its manufacture .

In accordance with the present invention, the content of C and Al is reduced and an appropriate amount of Ti is introduced, which promotes intragranular transformation, and in addition, an appropriate amount of B is added to improve hardenability, while the hardenability parameter is controlled by the carbon equivalent Ceq and the weldability parameter by the sensitivity parameter cracking Pcm to optimal intervals and create a fine-grained structure of the base material and HAZ, containing mainly bainite, yes e Mo content with limited and due intragranular bainite formed using oxides of Ti as nuclei of crystallization, due to increased grain size of the HAZ, i.e. reducing the actual grain size improves the low-temperature viscosity of the HAZ, so as to obtain a high-strength welded steel pipe for the pipeline, which has an increased thickness. The invention consists in the following.

(1) A high-strength welded steel pipe for a pipeline having excellent low temperature viscosity, representing a steel pipe obtained by seam welding of the base material of the steel sheet, which is given the shape of the pipe, and the specified base material of the steel sheet contains the following components, in wt.%: C: from 0.010 to 0.050%, Si: from 0.01 to 0.50%, Mn: from 0.50 to 2.00%, S: from 0.0001 to 0.0050%, Ti: from 0.003 to 0.030%, O: from 0.0001 to 0.0080%, and B: from 0.0003 to 0.0030%, with a limiting content of: P to 0.050% or less, Al to 0.020% or less, and Mo to less than 0.10 %, and the rest: jelly about and inevitable impurities, while the Ceq value obtained from the following formula 1 is from 0.30 to 0.53, the Pcm value obtained from the following formula 2 is from 0.10 to 0.20, and the metal structure of the specified base the material of the steel sheet consists of polygonal ferrite, which occupies 20% of the area or less, and bainite, which occupies 80% of the area or more, the actual grain size in the main material being 20 μm or less, and the actual grain size in the heat affected zone of the weld is 150 μm or less:

where C, Si, Mn, Ni, Cu, Cr, Mo, V and B denote the content of individual components (in wt.%).

(2) A high strength welded steel pipe pipe having excellent low temperature viscosity according to paragraph (1), characterized in that the thickness of the base material of the steel sheet is from 25 to 40 mm.

(3) A high strength welded steel pipe pipe having a low temperature viscosity according to (1) or (2), characterized in that the tensile strength of said base material of the steel sheet in the circumferential direction of said steel pipe is from 600 to 800 MPa .

(4) High-strength welded steel pipe for piping having excellent low temperature viscosity according to any one of (1) to (3), characterized in that said base material of the steel sheet further comprises, in wt.%, One or both of the following components: Cu: from 0.05 to 1.50% and Ni: from 0.05 to 5.00%.

(5) A high-strength welded steel pipe pipe having excellent low temperature viscosity according to any one of (1) to (4), characterized in that said base material of the steel sheet further comprises, in wt.%, One or more of the following components : Cr: 0.02 to 1.50%, V: 0.010 to 0.100% Nb: 0.001 to 0.200%, Zr: 0.0001 to 0.0500%, and Ta: 0.0001 to 0.0500%.

(6) A high strength welded steel pipe for a pipeline according to any one of (1) to (5), characterized in that said base material of the steel sheet further comprises, in wt.%, One or more of the following components: Mg: from 0 , 0001 to 0.0100%, Ca: from 0.0001 to 0.0050%, REM: from 0.0001 to 0.0050%, Y: from 0.0001 to 0.0050%, Hf: from 0.0001 up to 0.0050%, Re: from 0.0001 to 0.0050%, and W: from 0.01 to 0.50%.

(7) A high strength welded steel pipe pipe having excellent low temperature viscosity according to any one of (1) to (6), characterized in that the weld metal contains, in wt.%: C: from 0.010 to 0.100%, Si : 0.01 to 0.50%, Mn: 1.0 to 2.0%, Al: 0.001 to 0.100%, Ti: 0.003 to 0.050%, and O: 0.0001 to 0.0500 %, with a limiting content of: P to 0.010% or less and S to 0.010% or less, and the rest: iron and inevitable impurities.

(8) A high strength welded steel pipe pipe having excellent low temperature viscosity according to paragraph (7), characterized in that said weld metal further comprises, in wt.%, One or all of the following components:

Ni: 0.2 to 3.2% and

Cr + Mo + V: 0.2 to 2.5%.

(9) A method of manufacturing a high-strength welded steel pipe for a pipeline having excellent low temperature viscosity, characterized by the manufacture of steel, in which Si and Mn are introduced for preliminary deoxidation, then Ti is introduced so that the composition of the steel matches the compositions according to any one of (1) and ( 4) - (6), then the steel is cast and the resulting steel slab is subjected to hot rolling, then the resulting steel sheet is shaped into a pipe and seam welding of the butt sections is carried out.

(10) A method of manufacturing a high-strength welded steel pipe for a pipeline having excellent low temperature viscosity according to paragraph (9), characterized in that said steel slab is heated to a temperature of 1000 ° C or higher, it is hot rolled with a relative compression of 2.5 or more in the region of temperatures preceding recrystallization, and water cooling is stopped at a temperature of 600 ° C. or lower.

(11) A method of manufacturing a high-strength welded steel pipe for a pipeline having excellent low temperature viscosity according to (9) or (10), characterized in that said steel sheet is shaped into a pipe during the UO process, the butt sections of the inner and outer surfaces of the pipe are welded with application of submerged arc welding, using sintered or fused type welded wire and flux, followed by rolling of the pipe.

(12) A method for manufacturing a high strength welded steel pipe for a pipeline having excellent low temperature viscosity according to (11), characterized in that the heat input during said submerged arc welding is from 4.0 to 10.0 kJ / mm.

(13) A method for manufacturing a high strength welded steel pipe for a pipeline having excellent low temperature viscosity according to any one of (9) to (12), characterized in that the weld zones are subjected to heat treatment.

(14) A method for manufacturing a high-strength welded steel pipe for a pipeline having excellent low temperature viscosity according to (13), characterized in that the weld zone is subjected to heat treatment in a temperature range from 300 to 500 ° C.

Brief Description of the Drawings

The drawing shows a schematic view of the structure of a reheated HAZ.

The implementation of the invention

The present invention provides a welded steel pipe made of a steel material with a low C content and a metal structure subjected to low-temperature transformation and containing mainly bainite to improve the viscosity, in which, while limiting the Mo content, the optimum ranges of the hardenability parameter Ceq and weldability parameter Pcm and to improve hardenability add B and due to intragrain bainite, in particular, they reduce the actual grain size in the HAZ and improve t low temperature viscosity. Thus, the main distinguishing features of the present invention are: reduction of Al content, regulation of oxygen content and the introduction of an appropriate amount of Ti to disperse small inclusions that are extremely effective as nuclei for intragranular transformation in the structure of the base material of the steel sheet, and their use as nuclei for intragranular transformation to reduce the actual grain size. It should be noted that in the following description, the base material of the steel sheet will also be referred to simply as “steel sheet”, and the welded steel pipe will also be referred to simply as “steel pipe”.

Intragranular bainite in the HAZ structure was obtained by converting intragranular ferrite, which was formed as a result of intragranular transformation occurring in steel at high temperature, using the above-mentioned fine inclusions as nuclei during cooling. According to the present invention, the establishment of optimal ranges of the hardenability parameter Ceq and weldability parameter Pcm is extremely effective for the formation of intragranular bainite in the HAZ structure of a steel pipe, while limiting the amount of added Mo. Due to the formation of intragranular bainite, the low-temperature viscosity of HAZ is significantly improved. Moreover, intragranular bainite can also contribute to suppressing the softening of the HAZ of a steel pipe.

The mechanism of the formation of intragranular bainite is believed to be as follows: anion-vacancy type oxides can retain large amounts of Mn ions. In addition, the MnS compound readily precipitates on oxides. Therefore, layers depleted of Mn are formed around oxides and sulfides. These Mn depleted layers act as nuclei in the transformation that occurs when the steel is heated to a high temperature, and the metal structure becomes austenitic, after which the steel is cooled. Usually, intragranular ferrite of a petal shape is formed. Intragrain ferrite has a high degree of subcooling at a high cooling rate or good hardenability. During cooling, intragranular ferrite is converted to bainite, which becomes intragranular bainite.

Typical anion-vacancy type oxides are finely divided oxides containing mainly Ti. When these oxides are used as nuclei, intragranular bainite of the petal form is formed. Then finely dispersed sulfides, mainly containing Mn, are precipitated together with finely dispersed oxides, mainly containing Ti. It should be noted that, depending on the chemical composition of the steel, sometimes oxides may include one or more of the components Al, Si, Mn, Cr, Mg, and Ca, and sulfides may include one or more of the components Ca, Cu, and Mg. The size of the inclusions forming the nuclei for intragranular bainite can be measured using a transmission electron microscope (TEM). Preferred is the size of the inclusions in the range from 0.01 to 5 microns.

When a large amount of intragranular bainite is formed in the HAZ structure, the mixture of martensite and austenite (the structural component of martensite-austenite, called “MA”) becomes finely dispersed in the place where the destruction begins, and therefore the low-temperature viscosity increases significantly. If the C content is reduced to 0.05% or less and fine inclusions are dispersed, during the formation of intragranular bainite, the intragranular structure becomes finer and the fracture portion during Charpy impact testing, i.e. actual grain size also becomes smaller. In addition, intragranular bainite is more durable than intragranular ferrite, and therefore the formation of intragranular bainite can suppress the softening of the HAZ.

In the central part of the thickness of the high-strength welded steel pipe for the pipeline in the HAZ (near the 1/2 thickness section, called the “1/2 section”), the coarse MA mixture, as shown schematically in FIG. 1, is re-present in the structure along the old austenite grain boundaries heated HAZ and becomes the starting point of fracture, while viscosity sometimes decreases. In figure 1, numeral 1 denotes the reheated HAZ, numeral 2 denotes a mixture of martensite and austenite, and numeral 3 denotes the old grain boundary of austenite. “Reheated HAZ” is a section of the weld metal and HAZ near the fusion line in the previous welding, reheated in the subsequent welding. Depending on the heat input during the welding, the HAZ may vary somewhat in size, however, usually it is a section within 10 mm of the reflow line. If cuts are made on test samples, for example, at a distance of 1 mm or 2 mm from the reflow line, then when tested for Charpy impact strength at a temperature of -40 ° C, the absorbed energy sometimes becomes less than 50 J.

The authors conducted intensive research to study the low temperature viscosity of the main material of the steel sheet and HAZ of a welded steel pipe and as a result found the following. For the formation of intragranular bainite in the HAZ structure, mainly finely dispersed Ti oxides, complex oxides and complex sulfides are effective, and, in addition, they are also useful for reducing the actual grain size of the base material. Due to this, the actual grain size in the HAZ structure may be 150 μm or less and the actual grain size in the structure of the base material of the steel sheet may be 20 μm or less.

In addition, if, when the Mo content is limited to less than 0.10%, the hardenability parameter is set using the carbon equivalent of Ceq from 0.30 to 0.53 and the weldability parameter is set using the cracking sensitivity parameter Pcm from 0.10 to 0.20, then ferrite in the structure of the main material of the steel sheet will occupy 20% of the area or less, and bainite will occupy 80% of the area or more, and the intragranular structure in the HAZ as a result of transformation becomes intragranular bainite. Due to this, the tensile strength of a welded joint made by seam welding reaches 600 MPa or more.

In particular, if the sheet has a thickness of 25 mm or more or 30 mm or more, the viscosity of the 1 / 2t portion of the base material of the steel sheet sometimes drops, but the structure of the base material of the steel sheet can be made finer, i.e. reduce the actual grain size due mainly to finely divided Ti oxides, complex oxides and complex sulfides. The reason is believed to be as follows. Firstly, when rolling is performed in the temperature range preceding recrystallization, this promotes the transformation at ordinary grain boundaries, and therefore the intragranular transformation on oxides, complex oxides and complex sulfides becomes difficult. It is believed that this is because if, as a result of rolling, the grain size becomes smaller than the grain size obtained by intragranular transformation, then the growth rate of bainite formed from the nuclei at the grain boundaries becomes too large. Thus, it is believed that before the intragranular transformation, the transformation at the grain boundaries ceases, being completed.

On the other hand, when the relative compression in the temperature range preceding recrystallization is insufficient, the grain size increases, in particular, in the central part along the sheet thickness, and therefore the growth of bainite formed at the grain boundary also slows down. Therefore, it is believed that the actual grain size decreases as a result of intragranular transformation, mainly on Ti oxides, complex oxides and complex sulfides. Moreover, it is believed that finely dispersed oxides act as bonding particles and inhibit the growth of crystalline grains, which is also effective in reducing the actual grain size in the structure of the base material of the steel sheet.

According to the present invention, in particular, even if the sheet thickness is 25 mm or more, it is possible to achieve that the actual grain size of the base material of the steel sheet is 20 μm or less. In order to ensure that polygonal ferrite in the structure of the steel sheet occupies 20% of the area or less and bainite occupies 80% of the area or more, when testing the specimen for Charlie impact strength at a temperature of -40 ° C, cut close to the surface, i.e. at a distance of about 2 to 12 mm from the surface of the steel material, absorbed energy of 200 J or more can be obtained. The absorbed energy when testing a Charpy impact specimen cut from a 1 / 2t section, i.e., essentially from the center of the sheet thickness, can be 100 J or more.

In the present invention, controlling oxygen content in steel production is extremely important for the formation of mainly finely divided Ti oxides, complex oxides and complex sulfides. In particular, when adjusting the chemical composition of steel, it is necessary for preliminary deoxidation to introduce Si and Mn in an amount in the above range, then to introduce Ti. The oxygen concentration in the steel upon introduction of Ti is preferably from 0.001 to 0.003%. Due to this, it is possible to disperse Ti oxides, in particular Ti ₂ O ₃ , to obtain a grain size of from 0.01 to 10 μm in an amount of 10 to 1000 / mm ² over an area of 1 μm ² . As a result, intragranular transformation and the structure of the main material of the steel sheet are stimulated, and the structure in the HAZ of the welded steel pipe becomes finer, i.e. actual grain size is reduced.

By adjusting the chemical composition during such a process of steel production, hot rolling a steel slab and achieving a relative compression of 2.5 or more, preferably 3.0 or more, in the temperature range from 900 ° C to the end of rolling, it is possible to obtain the actual grain size in the structure the base material of the steel sheet is 20 microns or less.

The actual grain size is taken to be the value obtained by imaging the structure during backscattering of electrons by transforming a region surrounded by boundaries having a crystal disorientation of 15 ° or more into a circle of equivalent diameter. Moreover, “polygonal ferrite” looks like white structures in the form of clusters that do not include coarse cementite, MA, or other coarse grains observed in the structure under an optical microscope. The structure of the main material of the steel sheet, observed under an optical microscope, contains polygonal ferrite and bainite, and the rest, sometimes including martensite, residual austenite, and MA.

In the present invention, bainite is defined as a structure during the formation of which carbides are deposited between plates or clusters of ferrite or carbides are deposited on plates. In addition, martensite is a structure during the formation of which carbides do not precipitate between plates or on plates. Residual austenite is austenite formed at high temperature, which is retained in the structure of the base material of a steel sheet or welded steel pipe.

Further, due to the heat treatment of the weld zone, the coarse MA mixture formed along the old grain boundaries of the HAZ austenite breaks up into finely dispersed cementite, thereby increasing the low-temperature viscosity. As a result of this, in the corresponding section 1 / 2t of the sheet or in the corresponding section +1 mm from it, low-temperature viscosity increases. For example, if the welding zone is heated to a temperature in the range from 300 to 500 ° C, then when tested for Charpy impact strength at a low temperature of -40 ° C of a sample with a V-shaped notch, the absorbed energy can be 50 J or more. Therefore, the material used at an extremely low temperature of -40 ° C or lower, in the structure of which intragranular bainite is formed, is preferably further subjected to heat treatment to obtain a mixed structure of intragranular bainite and cementite.

Below, the reasons for limiting the chemical composition of the base material of the steel sheet according to the present invention will be explained. It should be noted that the HAZ is a heat-affected zone that does not melt during welding, so the components that make up the HAZ are the same as in the main material.

C: C is an element that increases the strength of steel, but in the present invention, the content of C is limited in order to obtain a metal structure consisting mainly of bainite and to achieve both high strength and high viscosity. When the content of C is less than 0.010%, the strength of the material is insufficient. With a C content of more than 0.050%, a drop in viscosity is observed. Therefore, according to the present invention, the optimum C content is set in the range from 0.010 to 0.050%.

Si: Si is a deoxidizing element and is essential in the present invention. To achieve a deoxidation effect, it is required to introduce Si in an amount of 0.01% or more in steel. On the other hand, if the Si content is more than 0.50%, the HAZ viscosity drops, thus, the upper limit of Si is set to 0.50%.

Mn: Mn is an element used as a deoxidizing agent necessary to ensure the strength and toughness of the base material of the steel sheet, and in addition, Mn forms the MnS compound and other sulfides effective as nuclei for intragranular transformation. This is very significant in the present invention. To achieve these effects, it is necessary to introduce Mn in an amount of 0.50%, however, when the Mn content exceeds 2.00%, the HAZ viscosity decreases. Therefore, the range of Mn content is set from 0.50 to 2.00%. It should be noted that Mn is an inexpensive element, and therefore it is preferably introduced in an amount of 1.00% or more to ensure hardenability of steel. The optimal lower limit of the Mn content is 1.50% or more.

P: P is an impurity and when its content exceeds 0.050%, the viscosity of the base material of the steel sheet is significantly reduced. Therefore, the upper limit of the content of P is set at 0.050%. In order to increase the viscosity of the HAZ, the P content is preferably set to 0.010% or less.

S: S in the present invention is an important element for the formation of compounds of MnS and other sulfides effective as nuclei for intragranular transformation. If the S content becomes less than 0.0001%, the amount of sulphides formed falls and no noticeable intragranular transformation occurs, and therefore, the S content must be set to 0.0001% or more. On the other hand, if the base material of the steel sheet contains S more than 0.0050%, coarse sulfides are formed and the viscosity decreases, thus, the upper limit of the S content is set to 0.0050% or less. To increase the viscosity of the HAZ, the upper limit of the S content is preferably set to 0.0030% or less.

Al: Al is a deoxidizing additive, but in the present invention, in order to make Ti oxides finely dispersed, it is extremely important that the upper limit of the Al content is 0.020% or less. In addition, in order to facilitate intragranular transformation, the Al content should preferably be 0.010% or less. Moreover, a preferred upper limit of Al content is 0.008% or less.

Ti: Ti in the present invention is an extremely important element for the formation of Ti oxides, which are finely dispersed and effectively act as nuclei for intragranular transformation. However, with an excess Ti content, carbonitrides are formed, and therefore the viscosity decreases. Therefore, according to the present invention, the Ti content should be set from 0.003 to 0.030%. In addition, Ti is a strong deoxidizing agent; therefore, if Ti is introduced at a high oxygen content, coarse oxides are formed. For this reason, in the production of steel, it is necessary to deoxidize the steel in advance by introducing Si and Mn and reduce the oxygen content. If Ti oxides become coarser, intragranular transformation is hindered and the effect of fixing grain boundaries is weakened, and therefore the effective grain size in the structure of the base material of the steel sheet and HAZ of the welded steel pipe sometimes increases.

B: B is an element that causes an increase in hardenability if it is contained in a solid solution of steel, but if it is added in excess, a coarse compound BN is formed, which, in particular, causes a decrease in the viscosity of the HAZ, thus, the upper limit of the content of B is set to 0 , 0030%. According to the present invention, B is added to the material of the welded steel pipe in an amount of 0.0003% or more, which improves hardenability, and the hardenability parameter is adjusted in optimum ranges by means of the Ceq carbon equivalent and the weldability parameter by the cracking sensitivity parameter Pcm to provide strength and weldability. It should be noted that additive B in an amount of 0.0003% or more is also effective in inhibiting the formation of ferrite at grain boundaries. In addition, with the deliberate introduction of additive B, if a finely dispersed compound BN is formed, the solubility of N in the solid solution decreases and, along with this, the viscosity of the HAZ increases, so it is preferable to set the content of B to more than 0.0005%.

Mo: Mo is a useful element that improves hardenability, promotes the formation of intragranular bainite in the HAZ and, in addition, forms carbonitrides to increase strength, but adding it in an amount of 0.10% or more leads to an increase in the cost of the alloy. Therefore, according to the present invention, the content of expensive Mo is limited to less than 0.10%. In a welded steel pipe according to the present invention, the hardenability parameter is controlled in optimal ranges by means of the Ceq carbon equivalent and the weldability parameter is determined by the cracking sensitivity parameter Pcm in order to provide the necessary hardenability even with a reduced Mo content.

O: Oxygen is an element inevitably present in the composition of steel, but according to the present invention, it is necessary to limit the O content in the formation of oxides containing Ti. The oxygen content remaining in the steel during casting, i.e. the content of O in the main material of the steel sheet should be set in the range from 0.0001 to 0.0080%. The reason is that with an O content of less than 0.0001%, the amount of oxide particles is insufficient, while with an O content of more than 0.0080%, the amount of coarse oxide particles increases and the strength of the base material decreases and the viscosity of the HAZ decreases. In addition, if an increase in oxygen content leads to coarsening, mainly of Ti oxides, then the structure of the main material of the steel sheet and the structure of the HAZ of the welded steel pipe becomes coarser, i.e. actual grain size increases.

Moreover, as elements that increase strength and toughness, one or more of the following elements can also be added: Cu, Ni, Cr, V, Nb, Zr and Ta. Moreover, when the content of these elements is below the preferred lower limits, they do not have any adverse effect, thus, these elements can be considered impurities.

Cu and Ni: Cu and Ni are effective elements that increase the strength of steel, while there is no decrease in viscosity. To achieve this effect, the lower limit of the Cu content and the lower limit of the Ni content are preferably set to 0.05% or more. On the other hand, to suppress the formation of cracks during heating and welding of the steel sheet, preferably the upper limit of the Cu content is 1.50%. The upper limit of the Ni content is preferably 5.00%, since when it is excessive, the weldability deteriorates. It should be noted that Cu and Ni are preferably introduced as a mixture to suppress the formation of surface defects. In addition, in terms of cost, the upper limits of the contents of Cu and Ni are preferably set to 1.00% or less.

Cr, V, Nb, Zr and Ta: Cr, V, Nb, Zr and Ta are elements that form carbides and nitrides and increase the strength of the steel during dispersion hardening. You can enter one or more of these elements. To effectively increase the strength, the lower limit of the Cr content is 0.02%, the lower limit of the V content is 0.010%, the lower limit of the Nb content is 0.001%, and the lower limits of the Zr and Ta content are 0.0001%. On the other hand, with the excessive addition of Cr, due to the increase in hardenability, the strength increases and the viscosity sometimes decreases, thus, the upper limit of the Cr content is preferably set to 1.50%. In addition, with the excessive addition of V, Nb, Zr and Ta, the carbides and nitrides become coarser, as a result of which the viscosity sometimes decreases. Thus, the upper limit of the V content is preferably set to 0.100%, the upper limit of the Nb content is preferably set to 0.200%, and the upper limits of the Zr and Ta content are preferably set to 0.0500%.

In addition, in order to control the shape of the inclusions and improve the viscosity, one or more of the elements Mg, Ca, REM, Y, Hf, Re, and W can be added. In addition, if the content of these elements is below the preferred lower limits, they do not have any adverse effects. influences, therefore, these elements can be considered impurities.

Mg: Mg is an element effective in increasing the fineness of oxides and adjusting the shape of sulfides. In particular, in order to achieve the finely dispersed effect of Mg oxides, which act as nuclei for intragranular transformation and, in addition, suppress the increase in grain size, as bonding particles, Mg addition in the amount of 0.0001% or more is preferable. On the other hand, when Mg is added in an amount of more than 0.0100%, coarse oxides are formed and the viscosity of the base material of the steel sheet and HAZ of the welded steel pipe sometimes decreases. Thus, the upper limit of the Mg content is preferably set to 0.0100%.

Ca and REM: Ca and REM are elements suitable for adjusting the shape of sulfides and form granules to suppress the formation of the MnS compound elongated in the rolling direction, and, moreover, improve the characteristics of the steel material in the thickness of the sheet, in particular, increase the delamination resistance. In order to achieve these effects, lower limits for Ca and REM are preferably set to 0.0001% or more. On the other hand, if the upper limits of the Ca and REM contents are more than 0.0050%, the size of the oxides increases, the amount of finely dispersed oxides containing Ti decreases, and the intragranular transformation is sometimes inhibited, and therefore the preferred upper limit of the content of these elements is 0 , 0050% or less.

Y, Hf, Re, and W: Y, Hf, W, and Re are elements with effects that are inherent in Ca and REM. With their excessive addition, the intragranular transformation is sometimes inhibited. Therefore, the preferred ranges of the Y content, the Hf content and the Re content are respectively from 0.0001 to 0.0050%, and the preferred range of the W content is from 0.01 to 0.50%.

Further, according to the present invention, to ensure hardenability of the base material of the steel sheet and the HAZ of the welded steel pipe, it is achieved that the area occupied by bainite in the structure of the base material is 80% or more, and the formation of intragranular bainite in the structure of the HAZ, while the carbon equivalent Ceq, calculated according to the following formula 1, based on the content (in wt.%): C, Mn, Ni, Cu, Cr, Mo and V, is set in the range from 0.30 to 0.53.

Moreover, in order to ensure the low-temperature viscosity of the base material and the HAZ, the cracking sensitivity parameter Pcm calculated according to the following formula 2, based on the content (in wt.%): C, Si, Mn, Cu, Cr, Ni, Mo, V and In, set in the range from 0.10 to 0.20.

It should be noted that the selectively included elements Ni, Cu, Cr and V, when their content is below the aforementioned preferred lower limits, are impurities, therefore, in the above formulas 1 and 2, their content is entered as the value "0".

If in the metal structure of the base material of the steel sheet used for the welded steel pipe, bainite occupies 80% of the area or more, polygonal ferrite occupies 20% of the area or less, then the ratio of strength and toughness becomes good. In addition, if the formation of mainly Ti oxides results in an actual grain size of 20 μm or less, the viscosity of the base material of the steel sheet is improved. It should be noted that polygonal ferrite is also effective in order to make the structure of the base material of the steel sheet finer, i.e. with a smaller actual grain size. Preferably, the area occupied by polygonal ferrite is 3% or more. In addition, it is preferable that the thickness of the base material of the steel sheet is 25 mm or more, and it is preferable that the tensile strength in the direction corresponding to the direction along the circumference of the steel pipe is 600 MPa or more. This should prevent destruction due to internal pressure during pipeline operation. It should be noted that if an increase in internal pressure in the pipeline is necessary, the thickness of the base material of the steel sheet should preferably be 30 mm or more. On the other hand, it is preferable that the thickness of the base material of the steel sheet is 40 mm or less, and the tensile strength in the direction corresponding to the direction along the circumference of the steel pipe is preferably 800 MPa or less. This is due to the fact that with an increase in the sheet thickness and an increase in the tensile strength, the force increases when the basic material of the steel sheet is shaped in the UO process. It should be noted that usually the “direction corresponding to the direction along the circumference of the steel pipe” is the direction along the width of the base material of the steel sheet.

Next, a method for manufacturing a steel pipe will be explained.

After the manufacture of steel, as a result of the aforementioned manufacturing process, a steel slab is cast from steel. Casting can be performed in the usual way, but from the point of view of performance, continuous casting is preferred. The hot-rolled steel slab is heated.

For hot rolling, the heating temperature is set to 1000 ° C. or more. This is done in order to perform hot rolling at a temperature at which austenite is the only phase in the steel structure, i.e. in the austenitic region, and in order to make the structure of the base material of the steel sheet finer, i.e. with a smaller grain size. The upper temperature limit is not limited, but to suppress the growth of the actual grain size, it is preferable to set the reheating temperature of 1250 ° C. or lower.

Hot rolling can be carried out immediately after removing the casting from the heating furnace, so that the initial temperature of the hot rolling is not particularly limited. In order to reduce the actual grain size of the base material of the steel sheet, it is preferable to obtain a relative compression of the preform of 2.0 or more in the recrystallization region at a temperature above 900 ° C. The relative compression in the recrystallization region is the ratio of the thickness of the steel ingot to the thickness of the sheet at a temperature of 900 ° C.

Further, if the relative reduction is 2.5 or more in the region preceding recrystallization at a temperature of 900 ° C or lower, then after water cooling the actual grain size of the base material of the steel sheet is 20 μm or less. To reduce the actual grain size of the base material of the steel sheet, it is more preferable to obtain a relative compression of 3.0 or more in the region preceding recrystallization at a temperature of 900 ° C. or lower. It should be noted that according to the present invention, the phrase "relative reduction during rolling in the region preceding recrystallization" means the ratio of the thickness of the sheet at a temperature of 900 ° C to the thickness of the sheet after completion of rolling. In addition, the upper limits of relative compression in the region prior to recrystallization and in the field of recrystallization are not limited, but if we consider the thickness of the steel ingot before rolling and the thickness of the base material of the steel sheet after rolling, the upper limits of relative compression are usually 12.0 or less.

The final rolling temperature is preferably the temperature at which austenite is the only phase or higher temperature in the structure of the base material of the steel sheet during hot rolling. Thus, the final rolling temperature is preferably Ar ₃ or higher, however, a small amount of polygonal ferrite is formed, so that the final rolling temperature Ar ₃ can be set to 50 ° C. or higher. The values of Ac ₃ and Ar ₃ can be calculated based on the content (in wt.%) Of C, Si, Mn, P, Cr, Mo, W, Ni, Cu, Al, V and Ti:

Ac ₃ = 910-203√С-15.2Ni + 44.7Si + 104V + 31.5Mo + 13.1W-30Mn-11Cr-20Cu + 700P + 400Al + 400Ti

Ar ₃ = 910-310C-55Ni-80Mo-80Mn-15Cr-20Cu

Then, after rolling is completed, the sheet is cooled with water. If water cooling is completed at a temperature of 600 ° C. or less, the aforementioned metal structure is obtained and the base material of the steel sheet acquires excellent viscosity. The lower temperature limit at which water cooling is completed is not limited. Water cooling of the sheet can be performed until room temperature is reached, but taking into account productivity, and taking into account possible hydrogen-related defects, the preferred temperature for completing water cooling is 150 ° C. or higher. The composition of the steel according to the present invention includes components, including B, which increase hardenability, so after rolling, even when cooling in air, bainite is easily formed, but depending on the composition of the components and the heating temperature, polygonal ferrite and the area occupied by bainite are sometimes formed, becomes less than 80%.

To give the main material of the steel sheet a pipe shape, a steel sheet is shaped, preferably in the UOE process, using a C-press, U-press and O-press, followed by arc welding of the butt sections to produce a welded steel pipe.

For arc welding, from the point of view of the viscosity of the weld metal and productivity, it is preferable to use submerged arc welding. In particular, in the manufacture of a welded steel pipe having a thickness of 25 to 40 mm, it is preferable that the heat input during submerged arc welding on the inner and outer surfaces is from 4.0 to 10.0 kJ / mm. If the heat input is in this range, then intragranular bainite is formed in the HAZ in the welded steel pipe according to the present invention having the aforementioned composition of components, while the actual grain size in the HAZ becomes 150 μm or less and an excellent low temperature viscosity is achieved.

In particular, this is because, when performing submerged arc welding simultaneously in one pass along the inner and outer surfaces of the pipe, the heat input is less than 4.0 kJ / mm, then when performing tack welding before the main welding, sometimes between the inner metal surface and the outer metal surface of the pipe remains the weld metal. In addition, if the amount of heat input during submerged arc welding is 10.0 kJ / mm or less, then in a steel pipe, even with a thickness of 25 to 40 mm, an old austenite grain size of 500 microns or less. It is effective for increasing viscosity. It should be noted that the input heat during welding on the inner surface of the pipe and the input heat during welding on the outer surface of the pipe need not be the same. Some difference in the amount of heat input is also possible.

If with a thickness of a welded steel pipe from 25 to 40 mm, the amount of heat input during submerged arc welding on the inner and outer surfaces is from 4.0 to 10.0 kJ / mm, then during cooling of the HAZ from a temperature of 800 ° C to 500 ° With a cooling rate of 2 to 15 ° C / sec. Even at a lower than usual cooling rate of the welded steel pipe according to the present invention with the aforementioned chemical composition, intragranular bainite is formed in the HAZ, while the actual grain size in the HAZ becomes 150 μm or less and an excellent low-temperature viscosity is achieved.

In addition, to form the chemical composition of the weld metal, the wire used for welding preferably contains the following components, the range of which will be explained later, taking into account the dissolution of the components by the base material of the steel sheet. That is, the components include, in wt.%: C: from 0.010 to 0.120%, Si: from 0.05 to 0.50%, Mn: from 1.0 to 2.5%, and Ni: from 2, 0 to 8.5%, additionally contains Al: 0.100% or less, and Ti: 0.050% or less, and the rest: Fe and inevitable impurities. The content of B can be from 0.0001 to 0.0050% and one or more of the following components Cr, Mo and V can be included, the content of Cr + Mo + V being from 1.0 to 5.0%.

Next, the composition of the components of the weld metal will be explained.

C is an element extremely effective for increasing strength. The content of C is preferably 0.010% or more. However, if the C content is too high, cold cracks easily form in the weld. In particular, hardening sometimes occurs and the viscosity in the HAZ decreases in the so-called T-section, i.e. at the intersection of the zone of local welding with roller welding. Therefore, the upper limit of the content of C is preferably set to 0.100%. In order to improve the viscosity of the weld metal, an upper limit of the C content is preferably set to 0.050% or less.

Si is preferably included in an amount of 0.01% or more to prevent a weld defect comprising shells. On the other hand, with excessive introduction of Si, the low-temperature viscosity is significantly reduced, thus, the upper limit of the Si content is preferably set to 0.50% or less. In particular, sometimes when welding repeatedly, the low temperature viscosity of the reheated weld metal decreases, so it is preferable to set the upper limit of Si to 0.40% or less.

Mn is an element effective in providing an excellent strength to toughness ratio. The lower limit of the Mn content is preferably 1.0% or more. However, the introduction of Mn in large quantities promotes segregation. While the low temperature viscosity of the weld metal decreases, with a high Mn content, it is also difficult to manufacture the weld wire used for welding, so the upper limit of the Mn content is preferably set to 2.0% or less.

P and S are impurities. In order to reduce the adverse effect of these impurities on the low temperature viscosity and sensitivity to low temperature cracking of the weld metal, it is preferable to set upper limits for the content of these impurities of 0.020% and 0.010% or less. It should be noted that from the point of view of low temperature viscosity, a more preferred upper limit of the content of P is 0.010%.

Al is an element introduced in order to improve the melting and solidification of the metal in the manufacture of welding wire. In order to use finely dispersed Ti-based oxides to suppress the increase in grain size of the weld metal, it is preferable to introduce Al in an amount of 0.001% or more. However, Al is an element that promotes the formation of a mixture of MA, thus the preferred upper limit of the Al content is set to 0.100% or less.

Ti is an element that forms finely dispersed oxides, which serve as nuclei for intragranular transformation and contribute to an increase in the fine-grained structure, i.e. reducing the grain size of the weld metal. The introduction of Ti in an amount of 0.003% or more is preferred. On the other hand, with the introduction of Ti in large quantities, a large amount of Ti carbides is formed and the low temperature viscosity decreases, thus, the upper limit of the Ti content is preferably set to 0.050% or less.

Oh is an impurity. The amount of oxygen ultimately remaining in the weld metal is usually 0.0001% or more. However, when O remains above 0.0500%, the amount of coarse oxides increases and the viscosity of the weld metal sometimes drops, so the upper limit of the O content is preferably set to 0.0500% or less.

In addition, it is preferable to add, optionally, Ni, Cr, Mo, and V to the weld metal.

Ni is an element that improves hardenability and provides strength and, in addition, improves the low temperature viscosity of the weld metal. The introduction of Ni in an amount of 0.2% or more is preferred. On the other hand, if the Ni content becomes too large, hot cracks sometimes form, and therefore an upper limit of the Ni content has been set to 3.2% or less.

All elements Cr, Mo and V improve hardenability. To increase the strength of the weld metal, one or more of them may be introduced, the content being a total of 0.2% or more. On the other hand, if the content of one or more of Cr, Mo, and V in total exceeds 2.5%, the low temperature viscosity sometimes deteriorates, thus, the upper limit of the content is preferably set to 2.5% or less.

The weld metal may further comprise B.

B is an element that improves the hardenability of the weld metal. To increase strength, it is preferable to introduce B in an amount of 0.0001% or more. On the other hand, when the content of B is more than 0.0050%, the viscosity sometimes decreases, thus, the upper limit of the content of B is preferably set to 0.0050% or less.

The weld metal sometimes includes, due to dissolution, elements from the base material of the steel sheet, for example Cu, Nb, Zr, Ta, Mg, Ca, REM, Y, Hf, Re, W, etc., selectively added to the base material, and sometimes includes Zr, Nb, Mg and other elements added as necessary to improve the melting and solidification of the weld metal. They are inevitably impurities included.

To improve the roundness of the steel pipe after seam welding, the pipe can be rolled. To give roundness to the steel pipe during its rolling, it is necessary to ensure deformation in the plastic region, therefore, the degree of rolling of the pipe should be 0.7% or more. The degree of rolling of the pipe is the difference between the length of the outer circumference of the steel pipe after rolling and the length of the outer circumference of the steel pipe before rolling, divided by the length of the outer circumference of the steel pipe before rolling and expressed as a percentage. If the degree of rolling of the pipe is more than 2%, then sometimes plastic deformation of the base material and the welding zone leads to a decrease in the viscosity of the metal. Therefore, the degree of rolling of the pipe is preferably set to 0.7 to 2.0%.

In addition, the weld zone and the HAZ of the steel pipe are preferably heat treated. In particular, when heated to a temperature of 300 to 500 ° C, the coarse MA mixture formed along the old austenite grain boundaries decomposes into bainite and finely dispersed cementite, which improves viscosity. If the heating temperature is less than 300 ° C, sometimes the degree of decomposition of the coarse MA mixture is insufficient and the effect of improving viscosity is insufficient, thus, the lower temperature limit of the heat treatment is preferably set to 300 ° C or more. On the other hand, when the welding zone is heated to temperatures above 500 ° C, phases drop out and the viscosity of the weld metal sometimes deteriorates, and therefore the upper temperature limit of the heat treatment is preferably set to 500 ° C or less. If the MA mixture formed in the reheated HAZ decomposes into bainite and cementite, then, as seen by scanning electron microscope, the shape of these structural components is similar to the MA mixture, but there are small white precipitates inside, which makes it possible to distinguish these structural components from mixtures of MA.

The welding zone and HAZ can be subjected to heat treatment when the outer surface of the pipe is heated by a torch or when heated by high-frequency currents. The pipe can be cooled immediately after its outer surface reaches the temperature of the heat treatment, but at this temperature it is preferable to hold for 1 to 600 seconds to facilitate the decomposition of the MA mixture. However, taking into account the operating costs of the equipment and taking into account the performance of the process, the holding time is preferably set to 300 seconds or less.

EXAMPLES

The following are examples in accordance with the present invention.

Steels having the chemical composition shown in Table 1 were made. The oxygen concentration in the steel at the time Ti was introduced was controlled in the range from 0.001 to 0.003%. From these steels, steel castings of 240 mm thickness were made having the chemical composition shown in Table 1. These steel slabs were heated to the temperatures shown in Table 2 and hot rolled to obtain a thickness of 45 to 160 mm in the recrystallization region at a temperature of 950 ° C or higher. Next, hot rolling operations were performed in the region preceding recrystallization in the temperature range from 880 ° C to 800 ° C with relative compression, the magnitude of which is presented in table 2, in order to obtain sheets with a thickness shown in table 2. Temperature at the end the hot rolling operation of the sheet was Ar ₃ - 50 ° C or higher. Water cooling began at a temperature of 750 ° C and ceased at various temperatures.

From the obtained steel sheets, in accordance with JIS Z 2242 Standard, test specimens with a V-shaped notch were prepared in which the longitudinal direction is the direction along the width of the sheet, and the notch was made parallel to the direction along the thickness of the sheet. Charpy impact test pieces were cut from sections of the surface layer of the sheet, i.e. from portions located from a sheet surface at a distance of about 2 to 12 mm, and from 1 / 2t portions, i.e., essentially located in the central part along the sheet thickness. Charpy impact tests were carried out at a temperature of -40 ° C to determine the absorbed energy. The tensile properties of the material were evaluated on test samples in accordance with the American Petroleum Institute Standard. It should be noted that when forming the main material of a steel sheet having a sheet thickness of 25 to 40 mm, a small degree of deformation was obtained to obtain a welded steel pipe in the central part of the sheet thickness during forming, which was confirmed by analysis based on the finite element method. Additionally, the steel sheets were cold worked to make welded steel pipes, which were tested to determine the effect of strain hardening. As a result of this treatment, sometimes an increase in tensile strength (PR) of the material ranged from 20 to 30 MPa or so. The increase in strength had a negligible effect on the viscosity of both the central portion along the sheet thickness and the portion of the surface layer. The degree of this influence was within the measurement error.

The microstructures of the central sections along the thickness of the base material of the steel sheet were observed under an optical microscope, the areas occupied by polygonal ferrite and bainite were measured, and residual structures were confirmed, the actual grain size of the base material of the steel sheet was measured on the image of the structure during backscattering of electrons.

Welded wire, taking into account the dissolution of the main material of a steel sheet having the following chemical composition, in wt.%, C: from 0.010 to 0.120%, Si: from 0.05 to 0.5%, Mn: from 1.0 to 2, 5%, Al: 0.100% or less, and Ti: 0.050% or less, additionally containing, if necessary, Ni: from 2.0 to 8.5% and one or more of the elements Cr, Mo, V, with a Cr content + Mo + V: from 1.0 to 5.0%, and containing B: from 0.0001 to 0.0050%, and the rest: Fe and inevitable impurities, were used to obtain a weld by submerged arc welding, and with one heat input during welding at each internal and external sheet surface is from 4.0 to 10.0 kJ / mm. Further, some of the welds were heat treated at the temperatures shown in Table 2. It should be noted that samples were cut from the weld metal and their chemical composition was analyzed. The tensile strength of the weld metal was determined in accordance with JIS Standard Z 3111. The chemical composition and values of the tensile strength of the weld metal are presented in table 3.

Table 3 Item No. The chemical composition of the weld metal (in wt.%) Heat input Weld metal FROM Si Mn R S Al Ti ABOUT N Cr + Mo + V kJ / mm Strength (MPa) one 0,057 0.24 1,5 0.009 0.001 0.010 0.015 0.0160 0.5 6.4 720 2 0,045 0.18 1,6 0.008 0.002 0.011 0.013 0.0170 6.2 640 3 0,046 0.12 1.4 0.005 0.004 0.010 0.009 0.0190 7.0 650 four 0,045 0.10 1.9 0.007 0.002 0.009 0.010 0.0220 5.8 670 5 0,065 0.11 1.7 0.006 0.001 0.008 0.012 0,0230 5.5 640 6 0,075 0.21 1,6 0.007 0.001 0.008 0.013 0.0220 0.3 7.5 720 7 0,041 0.26 1,5 0.006 0.002 0.010 0.007 0.0250 1,0 6.5 750 8 0,046 0.30 1,6 0.005 0.001 0.008 0.014 0,0230 0.5 0.2 6.4 740 9 0,080 0.32 1.7 0.008 0.002 0.007 0.012 0.0240 6.2 630 10 0.056 0.17 1,5 0.006 0.002 0,021 0.018 0,0230 7.1 670 eleven 0,062 0.26 1,6 0.007 0.002 0.006 0.012 0.0190 0.7 5,6 740 12 0,052 0.21 1,6 0.008 0.002 0.006 0.010 0,0200 6.5 700 13 0,050 0.18 1.8 0.006 0.003 0.007 0.013 0.0220 6.4 690 fourteen 0,060 0.24 1,6 0.007 0.002 0.008 0.016 0.0240 6.6 690 fifteen 0,045 0.15 1,5 0.008 0.003 0.006 0.012 0,0200 6.2 650

Small samples were cut from welded joints. The actual grain sizes in the HAZ of the samples were measured on the image of the structure during backscattering of electrons. In addition, petal bainite, the formation of which begins on inclusions, was defined as intragranular bainite and the area it occupies in the structure of the material was measured. In addition, in accordance with JIS Standard Z 2242, V-notch test specimens cut from HAZ were tested for Charpy impact strength at -40 ° C and absorbed energy was determined. On the samples, V-shaped cuts were made in the main material at a distance of 1 mm from the penetration line. The tests were carried out at a temperature of -40 ° C. Additionally, samples were cut in accordance with the standard of the American Petroleum Institute, and the width direction perpendicular to the weld was made the longitudinal direction of the test specimen, the weld metal was actually the center of the specimen, and tensile tests were carried out to determine the fracture site. The test results are presented in table 4. In table 4, the metal structure of the base material formed during intragranular transformation is characterized by the relative area occupied by intragranular bainite.

It should be noted that some of the steel sheets were subjected to shaping during the UO process, submerged arc welding and rolling were carried out to obtain steel pipes, the microstructure and mechanical properties of which were studied. These studies have confirmed that the microstructure and mechanical properties of steel pipes are equivalent to the microstructures and mechanical properties of the base material of steel sheets and HAZ welded joints.

Products No. 1-9 are examples of carrying out the invention. In the main material of the steel sheet, the actual grain size was 20 μm or less, and in the HAZ the actual grain size was 150 μm or less. In addition, samples cut from the base material and from the HAZ, when tested for Charpy impact strength at a temperature of -40 ° C, showed absorbed energies in excess of 50 J and good low temperature viscosity. In these embodiments of the invention, when tensile tests of welded joints were carried out, failure occurred on the base material of the steel sheet, therefore, softening of the HAZ does not create problems. It should be noted that article No. 9 is an embodiment of the present invention in which the heat treatment temperature was lower than the preferred heat treatment temperature, and therefore its effect on improving the low temperature viscosity was somewhat less.

On the other hand, in articles No. 10, 11, 14 and 15, there were components of a base material of a steel sheet whose contents were outside the range of the present invention, while the manufacturing conditions of articles No. 12 and 13 were outside the range of the present invention. These examples are comparative. Among them, the product No. 10 had a high Al content, and the product No. 11 had a small Ti content; therefore, a decrease in the amount of intragranular bainite was observed in these products and, in addition, a drop in the low-temperature viscosity of the HAZ was observed.

Product No. 12 is an example in which the relative compression at a temperature of 900 ° C. or less was small, whereby the actual grain size of the base material of the steel sheet increased, and the low temperature viscosity of the base material of the steel sheet fell. In addition, product No. 13 is an example in which, after rolling the sheet, cooling was performed in air, therefore, the relative area occupied by polygonal ferrite in the structure of the base material increased and the strength decreased. Product No. 14 is an example in which Ceq and Pcm were low, and therefore the strength dropped. Product No. 15 is an example in which Ceq and Pcm were high, therefore, the strength was high and the viscosity of the base material of the steel sheet fell. In addition, during the tensile test, the fracture of the welded joint occurred in the HAZ, due to the fact that the main material of the steel sheet had high strength.

According to the present invention, even with a reduced Mo content in the steel, the low temperature viscosity of the HAZ of the welded steel pipe for the pipeline can be ensured, and an inexpensive high-strength welded steel pipe for the pipe with excellent low temperature viscosity and a method for manufacturing it can be provided. Additionally, according to the present invention, low temperature viscosity of a thick high strength welded steel pipe for a pipeline having a thickness of 25 mm or more or having a thickness of 30 mm or more can be provided. All this makes a significant contribution to the industry.

Claims (13)

1. High-strength welded steel pipe for the pipeline, which has excellent low temperature viscosity, obtained by seam welding of a steel sheet, which is given the shape of a pipe in which the sheet is made of steel containing the following components, wt.%:
FROM 0.010-0.050 Si 0.01-0.50 Mn 0.50-2.00 S 0.0001-0.0050 Ti 0.003-0.030 ABOUT 0.0001-0.0080 AT 0.0003-0.0030 R 0.050 or less Al 0.020 or less Mo less than 0.10 iron and inevitable impurities rest,
the carbon equivalent value of Ceq is from 0.30 to 0.53, the value of the cracking sensitivity parameter Pcm is from 0.10 to 0.20, and the structure of the steel sheet consists of polygonal ferrite, occupying 20% of the area or less, and bainite occupying 80% of the area or more, the actual grain size in the main steel material being 20 μm or less, and the actual grain size in the heat affected zone during welding is 150 μm or less,
where Ceq = С + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5,
Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B,
C, Si, Mn, Ni, Cu, Cr, Mo, V and B denote the content of individual components in wt.%. 2. The high-strength welded steel pipe according to claim 1, characterized in that the thickness of the steel sheet is from 25 to 40 mm. 3. The high-strength welded steel pipe according to claim 1 or 2, characterized in that the annular tensile strength of the steel pipe is from 600 to 800 MPa. 4. High-strength welded steel pipe according to any one of claims 1 and 2, characterized in that the sheet is made of steel, optionally containing one or both of the following components, wt.%:
Cu 0.05-1.50 Ni 0.05-5.00 5. High strength welded steel pipe according to any one of claims 1 and 2, characterized in that the sheet is made of steel, optionally containing one or more of the following components, wt.%:
Cr 0.02-1.50 V 0.010-0.100 Nb 0.001-0.200 Zr 0.0001-0.0500 That 0.0001-0.0500. 6. High strength welded steel pipe according to any one of claims 1 and 2, characterized in that the sheet is made of steel, optionally containing one or more of the following components, wt.%:
Mg 0.0001-0.0100 Sa 0.0001-0.0050 REM 0.0001-0.0050 Y 0.0001-0.0050 Hf 0.0001-0.0050 Re 0.0001-0.0050 W 0.01-0.50 7. High strength welded steel pipe according to any one of claims 1 and 2, characterized in that the weld metal contains, wt.%:
FROM 0.010-0.100 Si 0.01-0.50 Mn 1.0-2.0 Al 0.001-0.100 Ti 0.003-0.050 ABOUT 0.0001-0.0500 R 0.010 or less S 0.010 or less iron and inevitable impurities rest 8. The high-strength welded steel pipe according to claim 7, characterized in that the weld metal additionally contains, wt.%:
Ni 0.2-3.2 and / or Cr + Mo + V 0.2-2.5 9. A method of manufacturing a high-strength welded steel pipe for a pipeline having excellent low temperature viscosity according to any one of claims 1 to 8, wherein in the process of steel production, it is preliminarily deoxidized with Si and Mn, and then Ti is introduced to obtain a steel composition containing wt.%:
FROM 0.010-0.050 Si 0.01-0.50 Mn 0.50-2.00 S 0.0001-0.0050 Ti 0.003-0.030, ABOUT 0.0001-0.0080 AT 0.0003-0.0030 R 0.050 or less Al 0.020 or less Mo less than 0.10
if necessary, one or more of
Cu 0.05-1.50 Ni 0.05-5.00 Cr 0.02-1.50 V 0.010-0.100 Nb 0.001-0.200 Zr 0.0001-0.0500 That 0.0001-0.0500 Mg 0.0001-0.0100 Sa 0.0001-0.0050 REM 0.0001-0.0050 Y 0.0001-0.0050 Hf 0.0001-0.0050 Re 0.0001-0.0050 W 0.01-0.50 iron and inevitable impurities rest,
the resulting steel is cast into a slab, which is heated to a temperature of 1000 ° C or higher and hot rolled to produce a sheet with a relative compression of 2.5 or more in the region of previous recrystallization of temperatures of 900 ° C or lower, followed by water cooling, which is stopped at a temperature of 600 ° C or lower, after which the resulting sheet is shaped into a pipe and seam welding of butt sections from the inner and outer surfaces is performed. 10. The method according to claim 9, characterized in that the steel sheet is shaped into a pipe during UO shaping, the butt sections of the internal and external surfaces are welded using submerged arc welding using a welding wire and sintered or fused flux, after which the pipe is rolled . 11. The method according to claim 10, characterized in that the input heat during submerged arc welding is from 4.0 to 10.0 kJ / mm 12. The method according to claim 9, characterized in that the zone of the weld is subjected to heat treatment. 13. The method according to p. 12, characterized in that the zone of the weld is subjected to heat treatment in the temperature range from 300 to 500 ° C.

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